The present exemplary embodiment relates to a non-aqueous ink jet ink. It finds particular application in conjunction with the printing of offset and other lithographic master plates, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Lithographic printing has long been the most widely used printing technique, especially for short to medium printing run lengths of 1,000 to 15,000. By the term “lithographic” is meant to include various terms used synonymously, such as offset, offset lithographic, wet lithographic, planographic, and others.
Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, will not accept the ink. The areas, which accept ink, form the printing image areas and the ink-rejecting areas form the background areas. In the art of photolithography, a photographic material is made image-wise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.
In the production of common lithographic plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
To make a typical lithographic printing plate, a substrate is provided with a hydrophilic surface which is imaged with an oleophilic material corresponding to the pattern to be printed. The oleophilic material is typically a photosensitive photopolymer or diazo resin which is exposed to light by masking with a film negative. The exposed portions are rendered insoluble in a developer and the unexposed portions remain soluble. Insolubility is typically caused by cross-linking of the photopolymer, the rate of cross-linking enhanced by coating the photopolymer with an oxygen inhibition layer. After exposure the oxygen inhibition layer and the unexposed photopolymer are removed. Other negative working materials useful in this invention include diazo resins mixed with non-photosensitive polymers and resins.
The above described process is referred to as a negative working process because the unexposed material is removed. In a positive working process, the pattern to be printed is masked and the photosensitive exposed material is rendered soluble in a developer. Printing plates made according to either process are used on offset presses.
Presently, most offset printing plates are prepared by analog prepress techniques. Photo type-setters and conventional cameras containing silver halide paper and film generate positive texts and photographs which are eventually converted to film negatives for plate preparation. This process is labor intensive and costly. The computer has now made it possible to convert this prepress operation from an analog process to a digital process, thus eliminating substantial labor and raw material costs. Total digitization of information also makes it possible to eliminate the film negative step in plate preparation and go directly to the plate. There are several means of digitally imaging a plate directly. Two of the most promising methods use lasers or ink jet technology. Using lasers to image plates directly is very costly because of the sophisticated optics and their controls. Time is also a constraint with lasers because of the necessity for raster scanning. A further drawback to laser imaging is the need for expensive, high-speed plate coatings, most of which are commercially unproven.
Ink jet printing technology is much simpler and more cost effective. No optical beam deflectors or focusing optics are necessary. Ink jet heads can be easily integrated so that imaging time is minimized. The coatings on the offset plates needed in an ink jet plate maker can be standard negative working photopolymers with proven track records. High sensitivity silver halide coatings are not necessary.
In recent years, ink jet printers have replaced laser printers as the most popular hard copy output printers for computers. Some of the competitive advantages of ink jet printers have been low cost, reliability, and the ability to make color images without significantly increasing the cost of the printer. Both thermal ink jet and piezoelectric ink jet printing methods have been widely adopted for desktop computer printing. A third conventional type of ink jet printing, a continuous flow type method, has found acceptance in high quality color printing and proofing in graphics applications.
In light of the increased use of ink jet printers for producing lithographic printing plates, there is a continued need for durable, waterfast, and bleed resistant inks for use in such printers. More particularly, there is a continued need for a non-aqueous hydrophobic inks for use in the printing of lithographic printing plates.